Plant homeodomain fingers form a helping hand for transcription

Several recent publications demonstrate a co-activator function for a subgroup of plant homeodomain fingers, which, in humans, comprises PHF2, PHF8 and KIAA1718. Besides an N-terminal plant homeodomain (PHD), these proteins also harbor an enzymatically active Jumonji-C domain (JmjC). While they have been shown to bind via their PHDs to H3K4me3-bearing nucleosomes at active gene promoters, their JmjC-domains are able to remove mono-and dimethyl-lysine 9 or 27 on histone H3 or monomethyl-lysine 20 on histone H4, chromatin modifications that correlate with transcriptional repression. Such dual histone crosstalk ensures the proper removal of repressive histone marks following transcriptional activation by RNA polymerases I and II. Mutations in the PHF8 gene lead to X-linked mental retardation (XLMR) and knockdown of KIAA1718 and PHF8 homologs in zebrafish causes brain defects. Thus, the co-activator function of this new class of chromatin-modifying enzymes has important functional roles in neuronal development. To continue with the nomenclature for histone demethylases, we propose the usage of KDM7A, -B and -C for KIAA1718, PHF8 and PHF2 proteins, respectively.